serialization: add explanation of DiffIDs
serialization: add explanation of DiffIDs

DiffIDs and Manifest list digests were a bit confusing. Explain the
difference.

Fixes: #115
Signed-off-by: Brandon Philips <74d2b51265fa19d338eb593a7555ec2b932e59c0@coreos.com>

jonboulle/image-spec,mattspencer-arm/image-spec,xiekeyang/image-tools,opencontainers/image-spec,opencontainers/image-spec,xiekeyang/image-tools,vbatts/oci-image-spec,opencontainers/image-tools,mattspencer-arm/image-spec,vbatts/oci-image-spec,jonboulle/image-spec,opencontainers/image-tools